1. Field of the Invention
The present invention relates to a sensing device for detecting an approaching object such as a human body, which is useful for any of a switching circuit for electronic equipment, an additional function for display equipment and a touch sensor for electronic equipment, and further the approach sensing device is excellent in resistant to noise.
2. Description of the Related Art
A sensing device for detecting an approaching object (hereinafter referred to as approach sensing device) is a device that detects an approaching object such as a human body and notifies electronic equipment of the approaching object. With respect to a detection method of such an approach sensing device, various detection methods such as detecting infrared radiation emitted from a human body and applying the Doppler effect of a microwave have been available for such an approach sensing device.
One of approach sensing devices adopting such detection methods, there exists an approach sensing device of an electrostatic capacitance type, which uses change of capacitance of a capacitor constituted by a pair of detection electrodes in response to an approaching object such as a human body to the pair of detection electrodes. Such an approach sensing device of the electrostatic capacitance type can detect an object approaching a relatively narrow area, so that a plurality of sensing devices can be disposed on an insulation panel of electronic equipment. In this connection, such an approach sensing device of the electrostatic capacitance type is widely used in electronic equipment as an inputting device that alternates a mechanical switch of the electronic equipment. Because a switch contact of a mechanical switch is gradually worn out in accordance with repeated turning on and off operations. However, wearing of switch contact never occurs in an approach sensing device of the electrostatic capacitance type.
Further, various kinds of circuitry are available for an approach sensing device of the electrostatic capacitance type. As one example of such circuitry, there exists one circuitry that makes use of a communication signal, wherein the communication signal changes in response to an approaching object such as a human body. In the circuitry, a detection source signal having a prescribed frequency is supplied to one of a pair of detection electrodes, and the detection source signal is transmitted to another detection electrode of the pair of detection electrodes. Then the transmitted detection source signal is changed in response to an approaching object and outputted as a communication signal.
The Japanese publication of unexamined patent applications No. 2001-203565 disclosed an approach sensor that detected an approaching object such as a human body.
FIG. 12 is a schematic diagram of a conventional sensing device for detecting an approaching object (hereinafter referred to as approach sensing device) according to the Japanese publication of unexamined patent applications No. 2001-203565.
According to the Japanese publication of unexamined patent applications No. 2001-203565, as shown in FIG. 12, a phase of a measuring signal “Is”, which is outputted from an oscillation circuit 10, is shifted in response to an object such as a human body approaching to a detecting plate 22, that is, an approach sensor of the electrostatic capacitance type. Then the measuring signal “Is” is outputted as an output signal “IL”. Phase difference between the output signal “IL” and the measuring signal “Is” is compared by a DBM (double balanced modulator) or a phase detector 30.
Accordingly, approach of an object such as a human body can be detected.
However, in the above-mentioned approach sensing device of the electrostatic capacitance type, which makes use of the output signal “IL” (communication signal) that is obtained from the measuring signal “Is” (detection source signal) that is supplied to one of a pair of detecting electrodes and changes in response to an approaching object such as a human body, if an AC electric noise generated in electric equipment such as a TV display or a radio wave emitted from an aeronautical radio station or a maritime radio station exists in the surrounding of the approach sensing device, then such an AC electric noise or a radio wave is possibly transmitted to the detection electrodes through a human body as an interfering signal.
Accordingly, the approach sensing device may not be able to detect change of the communication signal although a human body approaches, and results in miss-operation.
Although the Japanese publication of unexamined patent applications No. 2001-203565 fails to teach countermeasure for such an interfering signal, it is commonly known that a band pass filter is provided in a transmission line for a communication signal so as to eliminate an interfering signal from a communication signal.
In this connection, relatively simple circuitry can eliminate an interfering signal in case a frequency of the interfering signal is sufficiently away from that of a detection source signal. However, in case a frequency of the interfering signal is close to that of the detection source signal, a band pass filter having narrow bandwidth in complex circuitry is essential for eliminating such an interfering signal having a frequency close to that of the detection source signal.
Accordingly, there exist problems such that a frequency of a detection source signal and a pass band of a band pass filter are hardly controlled or adjusted, and that designing the circuitry results in difficult.
Further, a number of component parts constituting a band pass filter in narrow bandwidth increases. Particularly, in case a plurality of approach sensing devices is disposed in a limited area, it creates another problem that a scale of circuit increases and cost of component parts rises, and further space for component parts on a circuit board is strained.